Discovery of new biosynthetic pathways: the lipid A story

Abstract

The outer monolayer of the outer membrane of Gram-negative bacteria consists of the lipid A component of lipopolysaccharide (LPS), a glucosamine-based saccharolipid that is assembled on the inner surface of the inner membrane. The first six enzymes of the lipid A pathway are required for bacterial growth and are excellent targets for the development of new antibiotics. Following assembly, the ABC transporter MsbA flips nascent LPS to the periplasmic side of the inner membrane, whereupon additional transport proteins direct it to the outer surface of the outer membrane. Depending on the bacterium, various covalent modifications of the lipid A moiety may occur during the transit of LPS to the outer membrane. These extra-cytoplasmic modification enzymes are therefore useful as reporters for monitoring LPS trafficking. Because of its conserved structure in diverse Gram-negative pathogens, lipid A is recognized as foreign by the TLR4/MD2 receptor of the mammalian innate immune system, resulting in rapid macrophage activation and robust cytokine production

Crosstalk between the lipopolysaccharide and phospholipid pathways during outer membrane biogenesis in Escherichia coli

“…LPS, on the other hand, contains three distinct components: lipid A, core oligosaccharides, and O-antigen (7,8). Lipid A is the sole essential component of LPS, and its biosynthesis involves nine enzyme-catalyzed reactions (8). The lipid A pathway has been widely investigated, and we recently produced a pathway model that incorporates all of the known regulatory mechanisms (9).…”

“…Phospholipids consist of a glycerol molecule, a phosphate group, and two fatty acid moieties (except for cardiolipins) (5) (see reviews (5,6) and SI Appendix for the biosynthesis and regulation of phospholipids). LPS, on the other hand, contains three distinct components: lipid A, core oligosaccharides, and O-antigen (7,8). Lipid A is the sole essential component of LPS, and its biosynthesis involves nine enzyme-catalyzed reactions (8).…”

Synthesis, Structure, and SAR of Tetrahydropyran-Based LpxC Inhibitors

“…7 LpxC is a cytosolic zinc metallo-enzyme that catalyzes the deacetylation of UDP-3-O-(acyl)-N-acetylglucosamine, the first nonreversible step in the biosynthesis of lipid A, the main component and the substrate to which LPS is attached in the outer membrane of most Gramnegative bacteria. 8,9 Because the biosynthesis of lipid A is essential for maintenance of the outer membrane, LpxC is an attractive target for the development of new agents.A number of inhibitors of LpxC have been reported in the literature. 10−15 One of the most well documented inhibitors is CHIR-090 (1, Figure 1).…”

Minor Modifications to the Phosphate Groups and the C3′ Acyl Chain Length of Lipid A in Two Bordetella pertussis Strains, BP338 and 18-323, Independently Affect Toll-like Receptor 4 Protein Activation

“…5), which is the first enzyme of the Raetz lipid A biosynthesis pathway (11). LpxA is an essential enzyme and catalyzes the reaction that adds the C3 acyl chain onto UDP-GlcNAc, and this C3 acyl chain can then become the C3Ј acyl chain, when two GlcN backbone subunits are joined by LpxB, to form the di-GlcN moiety of lipid A (11). Amino acid 173 is a serine in BP338 LpxA and a leucine in 18-323 LpxA.…”

“…Previous work in E. coli suggests that a specific region of the enzyme LpxA, dubbed the "hydrocarbon ruler" region, plays a role in determining the length of the C3 and C3Ј acyl chains in lipid A (9,10). LpxA is the first enzyme in the Raetz lipid A biosynthesis pathway, and it is responsible for the addition of the C3 acyl chain onto the N-acetyl glucosamine (GlcNAc) precursor molecule (11). In B. pertussis LOS, LpxA has been shown to play a role in the flexibility of the C3Ј acyl chain length (12).…”